The present invention relates to a laminated radiation member, a power semiconductor apparatus, and a method for making the same.
A known power semiconductor apparatus is, for example, one which is composed of the main part as shown in FIG. 4. In FIG. 4, 101 indicates a power semiconductor apparatus, 102 indicates a semiconductor chip comprising IGBT or the like, 103 indicates a metal base plate for radiating the heat generated from the semiconductor chip 102, 104 indicates a ceramic plate comprising aluminum nitride or the like for insulating the semiconductor chip 102 from the metal base plate 103, 105a indicates a first metal electrode provided above the upper surface of the ceramic plate 104, 105b indicates a second metal electrode provided below the lower surface of the ceramic plate 104, 106a indicates a first hard solder paste for bonding the ceramic plate 104 to the first metal electrode 105a, 106b indicates a second hard solder paste for bonding the ceramic plate 104 to the second metal electrode 105b, 107a indicates a first solder for bonding the semiconductor chip 102 to the first metal electrode 105a, 107b indicates a second solder for banding the metal base plate 103 to the second metal electrode 105b, 108a indicates a first metal wire comprising aluminum to be connected to the semiconductor chip 102, 108b indicates a second metal wire comprising aluminum to be connected to the first metal electrode 105a, and 109 indicates a silicone gel which covers the semiconductor chip 102, the ceramic plate 104, the first metal electrode 105a and the second metal electrode 105b and seals them.
The conventional power semiconductor apparatus having the above construction is usually made by the following method. In the case of making the conventional power semiconductor apparatus 101, first, hard solder pastes, which are the first and second hard solder pastes 106a and 106b, are printed at a given thickness on both surfaces of the ceramic plate 104. Then, two metal electrodes, which are the first and second metal electrodes 105a and 105b, are put on the hard solder pastes printed on both the surfaces of the ceramic plate 104 and heat treated at a given temperature, for example, about 850° C., thereby bonding the first and second metal electrodes to both surfaces of the ceramic plate 104.
Thereafter, the ceramic plate 104, to both surfaces of which the metal electrodes are bonded is bonded, to the metal base plate 103 with a high-temperature solder (melting point: about 260° C.) which is the second solder 107b, and the semiconductor chip 102 is bonded with a low-temperature solder (melting point: about 150° C.), which is the first solder 107a, to both surfaces of the ceramic plate 104 on which the metal electrodes are bonded. A metal wire, which is the first metal wire 108a, is connected to the semiconductor chip 102 by wire bonding, and a metal wire, which is the second metal wire 108b, is connected to the metal electrode which is the first metal electrode 105a by wire bonding.
Usually, the metal base plate 103 on which the semiconductor chip 102, the ceramic plate 104, the first metal electrode 105a and the second metal electrode 105b, and the like are mounted is contained in a package. Silicone gel 109 is vacuum injected into the package and cured by heating, whereby the semiconductor chip 102, the ceramic plate 104, the first metal electrode 105a and the second metal electrode 105b, and the like are covered with the silicone gel 109 and sealed. In this way, the conventional power semiconductor apparatus 101 is made.
However, since the insulation substrate (104) and the metal electrodes (105a,b) are bonded with the hard solders (106a,b), cracks occur due to the difference in expansion coefficient between the insulation substrate, which has a low thermal expansion coefficient, and the hard solders and metal electrodes, which have high thermal expansion coefficients. Furthermore, since the insulation substrate (104) and the radiation plate (103) are connected with solder, there is the problem of high thermal resistance.
On the other hand, as an example of using no solder for bonding of an insulation plate and a radiation plate, JP-A-11-269577 proposes a method of forming a metal base composite material having a heat sink function by a chemical process utilizing a reaction between a ceramic dispersion material and a molten metal. This method suffers from the problem that since the molten metal is high-pressure injected into the ceramic dispersion material, expensive facilities are required, causing an increase of cost. There may be considered a means to carry out the reaction under impregnating the ceramic dispersion material with molten metal, but in this case, there is the problem that the penetrating speed is slow. In this method, the insulation substrate and the metal base composite material as a radiation plate are connected with a metal film or are connected with disposing a compound containing a firing aid for the insulation substrate at the bonded surface between the insulation substrate and the metal film, and therefore the thermal conductivity is better than the case of connecting with a solder. However, occurrence of cracks caused by difference in thermal expansion coefficient between the insulation plate of low thermal expansion coefficient and the metal film of high thermal expansion coefficient or the metal film provided with a compound containing firing aid for the insulation substrate cannot sometimes be avoided.
As an example of using no metallic radiation plate, there is, for example, an aluminum-silicon carbide composite material known as a metal ceramics composite material. This composite material is generally prepared by making a molded body (preform) of ceramic particles, ceramic fibers, whiskers, etc., then impregnating the preform with a molten metal and cooling it. As the method for impregnating with molten metal, there are various known methods such as a method based on powder metallurgy, a method according to high-pressure casting, e.g., die casting (JP-A-5-508350), a melt forging method (“Material,” Vol. 36, No. 1, 1997, pages 40–46), spontaneous penetrating method (JP-A-2-197368), etc.
On the other hand, as power semiconductor apparatuses, there are known, for example, those which comprise a semiconductor chip comprising IGBT or the like, a metal base plate of about 4 mm thick comprising copper or the like for radiating heat generated from the semiconductor chip, and a ceramic plate of about 0.6 mm thick comprising aluminum nitride or the like for insulating the semiconductor chip from the metal base plate. A first metal electrode of about 0.4 mm thick comprising copper or the like is bonded to the upper surface of the ceramic plate with a first hard solder of a given thickness. A semiconductor chip is bonded to the upper surface of the metal electrode with a solder of about 0.2 mm thick. A second metal electrode of about 0.2 mm thick comprising copper or the like is bonded to the under surface of the ceramic plate with a second solder of a given thickness. The under surface of the ceramic plate and the second metal electrode are bonded to the radiation plate with a solder or a hard solder.
However, there is a problem of low radiation property because the insulating ceramic substrate and the radiation plate are connected with a solder. Moreover, in the case of bonding the insulating ceramic substrate and the metallic heat sink material with a hard solder by active metal method or the like, cracks caused by thermal stress at the time of bonding occur on the side of the insulating ceramic substrate because of the great difference in thermal expansion coefficient between both the materials. Furthermore, a multi-layer type bonded body, which is bonded with a solder and provided with a stress relaxing layer by a means other than soldering, is low in endurance when exposed to thermal cycles of cooling-heating and, besides, increases in thermal resistance due to the increase of bonded interfaces which inhibits radiation. Moreover, since stress relaxation is conducted by employing a multi-layer structure, the number of production steps necessarily increases, and, as a result, this causes an increase of production cost. This is a serious problem.
On the other hand, as a method of bonding different members, the applicant of the present application disclosed in application JP-A-11-228245, utilizing an adhesive composition comprising ceramic fine particles and a hard solder and capable of reducing thermal stress. However, the object of the invention disclosed in the above application is to inhibit the decrease of bonding strength and the occurrence of cracks during the cooling operation, mainly after bonding, in making members by bonding the different members and need airtightness. The patent application makes no mention which suggests improvement of endurance in a use environment, such as increasing peeling resistance and effectively inhibiting cracking at bonded portions under severe thermal cycles, where high temperature-low temperature with cooling operation is repeated many times, in applications such as heat sinks, laminated radiation members and power semiconductor apparatuses. That is, of course, the application does not have descriptions suggesting that the products can function as a bonding layer of heat sinks, laminated radiation members and power semiconductor apparatuses.